The present invention relates to methods for electrical discharge machining of works in roll form, and more particularly the invention relates to an electrical discharge machining method for giving a satin finish to the outer surface of a work in roll form with a predetermined uniform roughness and a high efficiency.
Generally, the life of satinized rolls used on rolling mills is such that the satinized rolls can be used effectively only for one or two hours due to wear of the satinized roll surface, although this life time differs somewhat depending on the thickness, width, mechanical properties and rolling reduction or draft of material to be rolled, rolling mill performance, etc. As a result, there has existed a need for an electrical discharge machine which is capable of satinizing a set of rolls (two or four rolls) on a rolling mill at least in a period of 1 to 2 hours. It is also preferable that a single electrical discharge machine be capable of giving a satin finish to the rolls used on two or more rolling mills and its machining capacity must be increased further for this purpose.
In the past, a method of giving a satin finish to the surface of rolls by an electrical discharge machine has been proposed in which method, as shown in FIG. 1, an electrode group 12 having substantially the same total length as a roll 10 supported on roll supports 22 is arranged opposite to the roll 10, the roll 10 is rotated from a drive motor 18 through a drive transmission mechanism 20 while at the same time the electrode group 12 is moved at intervals by rotating a head traversing screw shaft 26 from a head traversing motor 24, and a desired working gap is maintained by a servomechanism 16, thus effecting the electrical discharge machining.
With this method, the electrode group 12 is composed of a large number of insulated and divided electrodes which are arranged substantially to the same extent as the roll 10 along the axis thereof, and each of the plurality of electrodes (see FIG. 2) 30 which are insulated and divided by insulators 28 has a cross-sectional shape constituting a parallelogram as shown by the cross-sectional view of FIG. 2. In this case, as is well known, discharge between the roll and the electrode tends to jump to the electrode end portions having a reduced sectional area in view of the discharge efficiency, and this tends to impede the formation of a uniform satin finish over the entire roll surface. As a result, the electrode group 12 is moved along the roll axis a predetermined distance which is less than the length of one electrode piece from time to time. However, the uniformity of satin finish on rolling mill rolls must be such that a streaky pattern even of a very small degree that cannot be seen by visual inspection, has a detrimental effect, making it unfit for use as a rolling mill roll, and while the pass-fail test is effected by chalk test or the like, in practice it is not an easy matter to determine the proper timing for moving the electrode group 12 from time to time so as to ensure the uniform satin finish at all times. Moreover, due to the fact that the electrode group 12 of substantially the same length as the roll body length is moved along the roll axis, on each end the electrode piece is deviated from the roll, and consequently the deviated electrode piece does not cause discharge, thus changing the electrode shape having substantially the same shape as the desired contour of the finished roll. In this case, if the electrode changed in shape is moved in the opposite direction, until the disfigured portion of the electrode piece is consumed so as to conform with the other portions having the proper shape, the other portions or the greater portion of the electrode piece fails to produce discharge. This has the effect of greatly deteriorating the electrical discharge machining capacity.
There is another method known in the art in which, as shown in FIG. 3, the forward end of an undivided single electrode 12' is partially conformed with the shape of the outer curved surface of a roll 10 and the electrode 12' is fed at a constant speed in the lengthwise direction of the roll 10. The electrode material is changed to make the electrode consumable, and in this way the electrical discharge machining is effected. However, due to the use of the single electrode 12', the machining capacity is not sufficient for the machining of works such as rolling mill rolls having a large work surface area and requiring a high degree of efficiency, and therefore this method has not been put in practical use as yet.